1. Field of the Invention
The invention relates to a method for coating a hot-rolled or cold-rolled steel flat product, containing 6-30% wt. Mn, with a metallic protective layer by hot-dip coating. When “steel flat products” are mentioned here, what are meant are steel strips and steel sheets or comparable rolled products.
2. Description of the Related Art
Steels with high manganese contents are basically notably suitable for use in the area of vehicle construction, in particular in car manufacture, because of their advantageous combination of properties consisting of high strengths of up to 1,400 MPa and extremely high elongations (uniform elongations up to 70% and elongations at break of 90%). Steels which are suitable for this intended use having high Mn contents of 6% wt. to 30% wt. are known, for example, from DE 102 59 230 A1, DE 197 27 759 C2 or DE 199 00 199 A1. Flat products produced from the known steels exhibit isotropic deformation behaviour at high strengths and, in addition, are also still ductile at low temperatures.
Set against these advantages, the disadvantage, however, is that high manganese-containing steels are prone to pitting and are only passivated with difficulty. This major proneness, compared to low-alloy steels, to locally admittedly limited, but intensive corrosion with the action of increased chloride ion concentrations, makes the use of steels belonging to the material group of high-alloy Mn steel sheets specifically in body construction difficult. In addition, high manganese-containing steels are prone to surface corrosion, which also limits the range of their applicability.
Due to the technological interest in these steels, in particular in the automotive industry, it is absolutely essential, therefore, to passivate the steel surface in the form of cathodic corrosion protection, for example by applying a metallic zinc or zinc-containing coating.
To counteract the problem of corrosion susceptibility, it was therefore proposed to coat steel flat products, produced from high manganese-containing steels, with a metallic protective layer which protects against corrosive attack, like a large number of other steel flat products provided for use in the area of car body construction.
Electrolytic zinc coating has proved suitable for this purpose. However, this way of applying a zinc coating is relatively involved from a procedural point of view. It should be added that there is the risk that the steel material will absorb amounts of hydrogen, by means of which its mechanical properties will be impaired.
In the commercial field, steel strips or sheets can be provided more cheaply and more simply from a procedural point of view by hot-dip coating with a metallic protective coating. With hot-dip coating, the flat product to be coated in each case is heated to a specific bath-entry temperature, at which it is then immersed in a melt bath. To adjust the layer thickness of the protective layer, the excess metal of the coating is subsequently wiped off the flat product. Hot-dip coating is, in practice, also called “hot-dip galvanizing” or “hot-dip aluminizing”, depending on the basis of the coating material processed in each case.
Practical attempts to provide steel strips with high manganese contents with a metallic, protective coating by hot-dip coating have, however, resulted in fundamental problems when wetting the products to be coated with the coating melt. These led to the coating not adhering sufficiently to the steel substrate, with the result that cracks and flaking of the coating appeared, in particular when high manganese-containing sheets coated in such a way were cold deformed.
The possibilities, known from the group of high-alloy but lower Mn content steels, of improving the wettability by applying an intermediate layer of Fe or Ni did not meet with the success desired in the case of steel sheets having at least 6% wt. manganese.
In DE 10 2005 008 410 B3, it was proposed to apply an aluminium layer onto a steel strip containing 6-30% wt. Mn before the last annealing process preceding the hot-dip coating. The aluminium adhering to the steel strip prevents the surface of the steel strip from oxidising during the annealing of the steel strip preceding the melt film coating. Subsequently, the aluminium layer, like an adhesive agent, causes the coating produced by the melt film coating to also then adhere tightly and all-over to the steel strip, if the steel strip itself provides unfavourable conditions due to its alloying. To this end, in the known method the effect is utilised of iron diffusing from the steel strip into the aluminium layer during the annealing treatment which necessarily precedes the melt film coating. In the course of the annealing, a metallic layer, essentially consisting of Al and Fe, consequently forms on the steel strip, which metallic layer is firmly bonded to the substrate formed by the steel strip.
Another method for coating a high manganese-containing steel strip, containing 0.35-1.05% wt. C, 16-25% wt. Mn, the remainder being iron and unavoidable impurities, is known from WO 2006/042931 A1. According to this known method, the steel strip made in such a manner is firstly cold rolled and subsequently annealed in a re-crystallizing way in an atmosphere which behaves in a reducing way in relation to iron.
The annealing parameters are thereby selected so that an intermediate layer, which essentially consists entirely of amorphous oxide (FeMn)O, appears on both sides of the steel strip and additionally an outer layer appears which consists of crystalline Mn oxide, wherein the thickness of the two layers is at least 0.5 μm. Practical tests have shown that even steel strips pre-coated in such an elaborate way do not have the adhesion to the steel substrate that is required for cold deformation.
In addition to the previously explained prior art, a method for hot-dip coating a hot-rolled steel plate which has a high tensile strength is known from JP 07-216524 A. In the course of this known process, the steel plate is firstly de-scaled, pickled and cleaned. Then it is weakly oxidised, in order to produce a thin iron oxide film on it which has a thickness of 500-10,000 Å. This iron oxide film is subsequently reduced to active metallic iron by reducing heating. The reducing heating is carried out such that a selective oxidation of Si and Mn in the steel and a concentration of these elements on the surface are avoided. For this purpose, the reducing heating is carried out under an atmosphere whose hydrogen concentration is regulated in the range from 3-25% vol., in such a way that it, on the one hand, has a reducing power which is sufficient to reduce the iron oxide and, on the other hand, however, a selective oxidation of Si and Mn does not happen.
Finally, from U.S. Pat. No. 5,677,005 a method is known for coating a cold rolled steel strip product containing 0.5-2.0% wt Mn with a metallic protective layer by hot dip galvanizing, in which the steel strip product, prior to entering the hot dip bath, undergoes a pickling process in order to remove the manganese oxide adhering to the steel strip product, wherein at higher Mn, Si or Cr contents it is proposed that the pickling is combined with a mechanical method for removing the oxide. The pickling bath can contain a sufficiently high concentration of hydrochloric or sulphuric acid in the conventional way, wherein as a typical example a hydrochloric acid concentration of 5% is mentioned. The dwell time in the pickling bath is typically between 5 and 60 seconds in the known method.
Additionally, from U.S. Pat. No. 5,810,950 it is known that for steels with higher Mn contents the oxide film adhering to them can be removed by pickling in a hydrochloric acid solution. However, this measure in this prior art is in no way linked to a subsequent hot dip galvanization to be carried out.
Despite the numerous proposals to be found in the prior art for improving the result of a hot-dip coating, it has been shown in practice that flat products can also therewith not be produced from high manganese-containing steels with the procedural simplicity and reliability of production required for a successful and commercial market-driven implementation and which are protected against corrosive attack with a quality corresponding to the requirements of secondary producers.
Against this background, the object of the invention was to specify a method with which, with increased productivity and reliability of production, flat products, consisting of steels having high manganese contents, can be reliably coated with a metallic protective layer which effectively protects against corrosive attack.